The present invention is concerned with a light-sensitive mixture composed of a binder and a light-sensitive compound, or a light-sensitive combination of compounds, which undergoes a change of solubility in an aqueous-alkaline solution upon exposure to light. The invention is also concerned with a light-sensitive copying material comprising a support and a light-sensitive layer which is comprised of the above-indicated mixture.
Mixtures of the above-mentioned type, which contain positive-acting o-quinone diazides as the light-sensitive compounds and which are preferably used in the production of printing plates, are disclosed, for example, in German Pat. No. 1,195,166 (corresponding to U.S. Pat. No. 3,201,239). These known mixtures contain phenol-formaldehyde novolaks or cresol-formaldehdye novolaks as the binders.
In Published European Patent Application No. 021,718 and No. 054,258, novolaks which have been prepared by reacting substituted resorcinol derivatives with aldehydes or ketones are described for use as binders in mixtures of the aforementioned type.
Planographic printing forms which have been prepared from light-sensitive planographic printing plates based on such mixtures have the advantage of yielding large print runs. The printing stencil of such a printing form, however, shows a certain susceptibility to the action of dampening solutions containing alcohol, which are now frequently used. As a consequence, the printing stencil is attacked and the print run is reduced. In addition, resistance to aqueous-alkaline developing solutions and gasoline solvents, of the kind employed during copying operations or on printing presses, is insufficient in some cases.
Print run and resistance to chemicals of such printing stencils may be further increased by baking at temperatures above 200.degree. C., but due to baking residues from the layer, which develop in the non-image areas, this operational step requires an additional pre-treatment or post-treatment of the printing forms and thus involves an additional expenditure of work.
Novolaks prepared from phenols and aldehydes or ketones, respectively, also have the disadvantage of a relatively high brittleness, which has an unfavorable effect, especially in the processing of comparatively thick copying layers, for example, photoresist layers.